With the recent development of digital signal processing technology, outdoor voice telephone communication with a mobile phone, in-vehicle hands-free voice telephone communication and hands-free operation using voice recognition have been spread widely. Since devices for carrying out these functions are often used under a very noisy environment, background noise as well as voice is input to a microphone, thereby bringing about deterioration of telephone communication voice and reduction in a voice recognition rate. Accordingly, to realize pleasant voice telephone communication and highly accurate voice recognition, a noise suppression device for reducing background noise mixed into an input signal is required.
As a conventional noise suppression method, a method is known which converts an input signal in the time domain to a power spectrum which is a signal in the frequency domain, calculates a suppression quantity for noise suppression by using the power spectrum of the input signal and a noise spectrum estimated separately from the input signal, carries out amplitude suppression of the power spectrum of the input signal using the suppression quantity obtained, and converts the power spectrum passing through the amplitude suppression and the phase spectrum of the input signal into the time domain to obtain a noise suppressed signal, for example (see Non-Patent Document 1).
The conventional noise suppression method calculates the suppression quantity from the ratio (SN ratio) between the power spectrum of voice and the estimated noise power spectrum. However, it is effective only under a condition in which the noise superposed on the input signal is somewhat steady in the time/frequency direction, but cannot calculate the suppression quantity correctly if noise which is unsteady in the time/frequency direction is input, offering a problem of producing artificial residual rasping noise called a musical tone.
As for the foregoing problem, a method is disclosed, for example, which makes the residual rasping noise less audible by adding an input signal (original sound) passing through an appropriate level adjustment to the output signal after the noise suppression (see Patent Document 1, for example).
As another method, a method is disclosed which sets a prescribed target spectrum in advance to carry out stable noise suppression, reduces the occurrence of musical noise with respect to unsteady noise by controlling the noise suppression quantity in such a manner that the residual noise spectrum approaches the target spectrum, thereby carrying out natural and stable noise suppression (see Patent Document 2, for example).